Heterogeneous nucleation is very important for the initial droplet formation of dropwise condensation, and it is directly related with the surface properties of materials. In this paper, the relationship of nucleation site density with surface topography in nanometer scale for dropwise condensation was experimentally studied on surfaces of magnesium in view of that magnesium can react with hot water (condensate) and make the chemical compositions of the surface changed, and thus leave the nucleation sites in nanometer on the surface. The differential box-counting was used to calculate the fractal dimension of these magnesium surfaces. And condensation experiments were carried out on the magnesium surfaces with different topography under same subcoolings and condensation time. The results showed that surface topography has a great influence on nucleation site density. The larger the fractal dimensions the more the nucleation sites. A new formula to calculate the nucleation site density was proposed based on the modification of Rose's formula with the experimental data. And this new formula can reflect the effect of surface fractal dimension on the nucleation sites. (c) 2007 Elsevier Ltd. All rights reserved.